An STM tip applies a voltage, initiating polymerization of the diacetylene groups in surface-bound carboxylic acid molecules (black and red). The polymerized wire (yellow) propagates until it bumps against a single phthalocyanine molecule (blue) in a cluster. A bond forms, and the completed "circuit" is shown in an STM image.

Credit: J. Amer. Chem. Soc.

A single-molecule electrical circuit, in which organic compounds substituting for components such as wires, transistors, and rectifiers are all covalently bonded, just took a step closer to reality, according to a new report (J. Am. Chem. Soc., DOI: 10.1021/ja111673x). In addition to being exceedingly small—a major goal in electronics—such a circuit could have higher computing power than current silicon-based devices.

Connecting molecular nanowires in a controlled fashion to single molecules that function like circuit elements is “one of the great challenges of the field,” says Mark A. Ratner, a materials chemist at Northwestern University. Yuji Okawa of Japan’s National Institute for Materials Science and coworkers have now addressed this challenge by developing “chemical soldering,” a bond-forming method that connects a conductive polymer wire and a phthalocyanine molecule to create a circuit.

The researchers precisely position the probe tip of a scanning tunneling microscope over the diacetylene segments of 10,12-nonacosadiynoic acid molecules assembled side by side on a flat graphite surface. An applied voltage initiates polymerization of the diacetylene moieties, forming a “wire” that propagates until it bumps against a nanocluster of phthalocyanine molecules. One of these molecules, deposited on top of the long carboxylic acid chains, forms a bond to the reactive end of the nanowire about 70% of the time, the researchers find.

An STM tip initiates polymerization of the diacetylene groups in long- chain carboxylic acid molecules on a surface to form a wire (yellow) that then forms a bond to the single phthalocyanine molecule (blue). R=CH3(CH2)15 and R´=HOOC(CH2)8.

Credit: J. Amer. Chem. Soc.

An STM tip initiates polymerization of the diacetylene groups in long- chain carboxylic acid molecules on a surface to form a wire (yellow) that then forms a bond to the single phthalocyanine molecule (blue). R=CH3(CH2)15 and R´=HOOC(CH2)8.

Credit: J. Amer. Chem. Soc.

With the chemical soldering technique, the researchers connect two polymer wires to a single phthalocyanine molecule. This circuit, they say, could act as a molecular-scale resonant tunneling diode, a circuit element that allows electrons to tunnel through at specific energy levels.

This work is a “breakthrough” for the field, says Robert Stadler, a physical chemist at the University of Vienna, but “it doesn’t mean that we are now ready to print single-molecule circuits.” Further studies must be done, and Okawa says that measuring the electronic properties of his new circuit and linking polymer nanowires to molecules other than phthalocyanine are up next.